This application is based on Patent Application Nos. 314,057/1997 filed on Nov. 14, 1997 in Japan and 318,085/1998 filed on Nov. 9, 1998 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing apparatus and a printing method that are applicable to any apparatuses for printing recording mediums, such as paper, cloth, nonwoven fabric and OHP, and more specifically to a printing apparatus and a printing method which are most suitable for an ink-jet printing apparatus installed in a variety of office equipment and mass-produced apparatuses such as printers, copying machines and facsimiles.
2. Description of the Prior Art
An ink-jet printing apparatus for printing recording mediums such as paper, cloth, plastic sheets and OHP sheets has been commercialized as an output means of information processing systems, for example as a printer for copying machines, facsimiles, electronic typewriters, word processors and workstations, or as a handy or portable printer used on personal computers, host computers, optical disk drives and video tape recorders.
The ink-jet printing apparatus in the above case has a structure corresponding to part of this type of equipment, or incorporated into their unique functions. Generally, the ink-jet printing apparatus comprises a carriage mounted with a recording means (print head) and an ink tank; a feeding means to feed print paper; and a control means to control the recording means and the feeding means. The print head ejects an ink from a plurality of nozzles and is serially scanned in a direction (hereinafter referred to as a main scan direction) perpendicular to the print paper feeding direction (a sub-scan direction) and at the same time the print paper is intermittently advanced by a distance equal to a recording width during non-printing periods. This method delivers ink onto the print medium according to a print signal to perform printing and has found a wide range of applications as a low-noise recording system with a small running cost.
Because the print head has many ink ejecting nozzles arranged in line in the sub-scan direction, scanning the print head over the print paper results in the paper being printed in a width corresponding to the number of nozzles, thus assuring high-speed printing.
In recent years, a device has been in use that includes a plurality of print heads for three to four colors to form full-color images. This device can mount four print heads and four ink tanks corresponding to three primary colorsxe2x80x94yellow (Y), magenta (M) and cyan (C)xe2x80x94and black (B).
A recent trend of the ink-jet printer is for reducing the size of ink droplets to enhance the printed image quality to cope with high-speed printing and photograph printing. The reduced size of ink droplets requires enhanced landing precision. It is, however, unavoidable that the actual landing points of the ink droplets are deviated from ideal landing positions.
To deal with this problem, Japanese Patent Laid-Open Nos. 214,670/1985 and 214,671/1985, and U.S. Pat. Nos. 4,963,882 and 4,967,203 offer a multipass recording method. In this recording method, the image data is divided into predetermined arrays for first and second scans so that the first-scan array and the second-scan array complement each other. Generally, the divided arrays of the pixels are like a hound""s-tooth check pattern in the vertical and lateral directions as shown in FIG. 1 and the material to be printed is fed intermittently when the printing is not performed. FIG. 1 represents pixels in the form of squares in a grid and the numbers indicate in which scan the ink will be ejected onto the corresponding pixels. With the multipass recording, because one line is formed of ink droplets ejected from different nozzles, variations that depend on the nozzles can be reduced.
There is also a printing apparatus which has a plurality of columns of ink ejecting nozzles to improve the printing resolution for higher image quality. In this case, the deviations between the ink droplet actual landing positions and the ideal landing positions differ from one nozzle column to another. In other words, the same nozzle column will produce the similar landing deviations. The inventor of this invention has found that if only the nozzles of the same nozzle column are used in each scan line to form an image, an unevenness of an image by the nozzle is reduced, but an unevenness of the image by the nozzle column is not reduced at all, and this results in large unevenness of an image during the quantization process, causing significant image quality degradation in some cases.
This is explained by taking an example shown in FIGS. 2 through 5. FIG. 2 shows a print head having columns of nozzles n1, n2, the nozzle column n1 ranging from nozzle n1-1 to nozzle n1-256 and the nozzle column n2 from nozzle n2-1 to nozzle n2-256. Let us consider a case where the nozzles n1-1 to n1-256 of the nozzle column n1 have ink landing points that tend to deviate to the right from ideal landing points and the nozzles n2-1 to n2-256 of the nozzle column n2 have ink landing points that tend to deviate to the left from ideal landing points. It is assumed that a print pattern as shown in FIG. 3 is printed by a 1-raster-2-pass multipass printing. With the division method used by this multipass printing, ink dots are thinned out to form a hound""s-tooth check pattern described above. FIG. 3 represents pixels in the form of squares in a grid and dots in the form of circles. When there are no deviations between the actual landing points and the ideal landing points, the image produced will have a uniform dot distribution as shown in FIG. 3. In the case under consideration, however, the image obtained will be as shown in FIG. 4. This is because the landing positions of odd-numbered lines of dots produced by the nozzle column n1 are deviated to the right on the sheet of the figure and because the landing positions of even-numbered lines of dots produced by the nozzle column n2 are deviated to the left. The directions of deviations of the landing points are shown in FIG. 5, from which it is seen that the deviation direction changes for each line.
Hence, when the multipass recording is performed by the print head which has a plurality of staggered columns of print elements, an actual printed image looks to have a slantwise texture, different from the one that is originally intended to be represented.
An object of this invention is to provide a printing apparatus and a printing method, which can form an image strictly according to what is intended to be represented by the image data even when a multipass printing is performed by a print head which has a plurality of staggered columns of print elements.
The present invention provides a printing apparatus using a print head. The printing apparatus may comprise a main scan means to move the print head relative to a recording medium in a main scan direction; a sub-scan means to move the recording medium relative to the print head in a sub-scan direction different from the main scan direction; and a control means to main-scan the print head a plurality of times over one and the same print line on the recording medium with at least one sub-scan operation interposed between the main scan operations to print the line, the print head having a plurality of staggered columns of print elements; wherein the control means uses the print elements of different columns to complete the printing of the same print line.
According to the invention, the print elements may be print elements for an ink-jet printing system.
According to the invention, odd-numbered columns of the print elements and even-numbered columns of the print elements may be staggered with respect to each other.
According to the invention, the control means may scan the print head two times over the same print line by interposing between the main scan operations the sub-scan operation which advances the recording medium by a distance equal to one-half the length of the print element columns plus or minus one-half the print element interval.
According to the invention, the control means may scan the print head two times over the same print line by interposing between the main scan operations the sub-scan operations which advances the recording medium by a distance equal to one fourth the length of the print element columns plus or minus one-half the print element interval.
According to the invention, the odd-numbered columns and the even-numbered columns of the print elements each may use an odd number of print elements for printing, and the control means may scan the print head two times over the same print line by interposing between the main scan operations the sub-scan operation which advances the recording medium by a distance equal to one-half the length of the print element columns.
According to the invention, the odd-numbered columns and the even-numbered columns of the print elements each may comprise four times an arbitrary odd number of print elements, and the control means may scan the print head four times over the same print line by interposing between the main scan operations the sub-scan operation which advances the recording medium by a distance equal to one fourth the length of the print element columns.
According to the invention, the odd-numbered columns and the even-numbered columns of the print elements each may comprise n times an arbitrary odd number of print elements, and the control means may scan the print head n times over the same print line by interposing between the main scan operations the sub-scan operation which advances the recording medium by a distance equal to one n-th the length of the print element columns where n is an arbitrary natural number other than 1.
According to the invention, the printing apparatus may be capable of color printing and have a plurality of columns of print elements for each print color.
According to the invention, the printing method using a print head to perform printing may comprise the steps of: moving the print head relative to a recording medium in a main scan direction; moving the recording medium relative to the print head in a sub-scan direction different from the main scan direction; and controlling to main-scan the print head a plurality of times over one and the same print line on the recording medium with at least one sub-scan operation interposed between the main scan operations to print the line, the print head having a plurality of staggered columns of print elements; wherein the print elements of different columns are used to complete the printing of the same print line.
According to the invention, the print elements may be print elements for an ink-jet printing apparatus.
According to the invention, odd-numbered columns of the print elements and even-numbered columns of the print elements may be staggered with respect to each other.
According to the invention, the control step may scan the print head two times over the same print line by interposing between the main scan operations the sub-scan operation which advances the recording medium by a distance equal to one-half the length of the print element columns plus or minus one-half the print element interval.
According to the invention, the control steps may scan the print head two times over the same print line by interposing between the main scan operations the sub-scan operations which advances the recording medium by a distance equal to one fourth the length of the print element columns plus or minus one-half the print element interval.
According to the invention, the odd-numbered columns and the even-numbered columns of the print elements may each use an odd number of print elements for printing, and the control step may scan the print head two times over the same print line by interposing between the main scan operations the sub-scan operation which advances the recording medium by a distance equal to one-half the length of the print element columns.
According to the invention, the odd-numbered columns and the even-numbered columns of the print elements may each comprise four times an arbitrary odd number of print elements, and the control step may scan the print head four times over the same print line by interposing between the main scan operations the sub-scan operation which advances the recording medium by a distance equal to one fourth the length of the print element columns.
According to the invention, the odd-numbered columns and the even-numbered columns of the print elements may each comprise n times an arbitrary odd number of print elements, and the control means may scan the print head n times over the same print line by interposing between the main scan operations the sub-scan operation which advances the recording medium by a distance equal to one n-th the length of the print element columns where n is an arbitrary natural number other than 1. This invention may be capable of color printing and have a plurality of columns of print elements for each print color.
According to the invention, because the printed dots representing one and the same line are formed by ink droplets ejected from the nozzles of different nozzle columns, the degree to which the dot landing deviations have a directivity in the line can be alleviated. As a result, when an image is formed by using nozzle columns whose ink droplet landing positions will deviate, it is possible to prevent unintended textures from appearing in the printed image.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.